faces semi-transparent in edit mode, revealing background plane
You don't need to have your reference image as a plane, you can also have it as a background which is visible in wireframe mode too. To do so you have to press N to open up the properties and then you have to scroll down to 'Background Images'. Check the box, click 'Add Image' and open up your Image. From there on you can modify the image the way you want like changing it's opacity or making it only visible from top view. 

Another way of doing this is to change the maximum drawing type of your car hood to 'Wire' in the 'Object' tab (orange cube, then scroll down to display). This makes only the car hood appear in wire mod and everything else is in the mode you want e. g. Textured, so you can see through your hood but you can still see your background plane textured.